The invention relates to a method of operating a heart pacemaker for the termination of a tachycardia and also relates to a heart pacemaker for the implementation of this method.
The unnaturally elevated frequency of the heartbeat, for example when the heart executes more than 180 beats per minute, is referred to as a tachycardia. It is known that this rapid heart rhythm can be stopped by means of electric stimulation pulses when it has been caused by retrograde transconduction. The prerequisite, however, is that a stimulation pulse must be emitted to the heart within a critical time interval after a tachycardia heartbeat, a so-called time window. This time window can lie immediately after the refractory time following a heartbeat or can lie only immediately before a next heartbeat. It is further known from investigations that individual stimulation pulses do not always suffice to effectively terminate a tachycardia but that sequences consisting of a plurality of stimulation pulses can be required. It can be necessary for terminating a tachycardia that the intervals between the individual pulses of a sequence differ in length. Investigations have further shown that the effective sequences can depend on the physical position of the patient, on physical activities, drugs and the like. Further, the required sequence can vary greatly from patient to patient. It is not possible for the above reasons to identify a fixed sequence for even one patient that can always reliably terminate a tachycardia.
A series of methods or, respectively, heart pacemakers are already known which are concerned with how the proper point in time for the stimulation can be found. Thus, U.S. Pat. 3,942,534 discloses a method wherein the time interval between the end of the refractory time and the next following tachycardia heartbeat is scanned in equidistant steps. Since the time window can be very small, the steps must be selected very small so that a sweep of the possible time interval can last very long under certain conditions.
A heart pacemaker is known from U.S. Pat. 4,312,356 wherein the reaction of the heart to a stimulation pulse is detected and employed for the control of the interval up to the next stimulation pulse such that the change of the interval is respectively carried out in a direction of the interval size effective for the termination of a tachycardia.
Known, finally, from U.S. Pat. 4,280,502 is a heart pacemaker wherein, upon occurrence of a tachycardia, the interval between the end of the refractory time and the next tachycardia heartbeat is likewise scanned until an effective stimulation for the termination of the tachycardia has been found. In addition, the refractory time can be identified in advance in a similar manner. The advantage of this known method and heart pacemaker is that the value, once found, can be stored in a register and, upon occurrence of a repeated tachycardia, can be employed as a first experimental value for the termination thereof.